Steel for hot working die, die for hot working, and manufacturing method for same

ABSTRACT

Provided are a steel that is for a die and that enables production of a die being for hot working and having both high hardness and high thermal conductivity; a die for hot working; and a manufacturing method for the same. The steel for a hot working die has a compositional makeup containing, in mass %, 0.45-0.65% of C, 0.1-0.6% of Si, 0.1-2.5% of Mn, 1.0-6.0% of Cr, 1.2-3.5% of (Mo+½W) where Mo and W are contained independently or in combination, 0.1-0.5% of V, 0.15-0.6% of Ni, 0.1-0.6% of Cu, and 0.1-0.6% of Al, the balance being Fe and inevitable impurities. Further, this die for hot working has said compositional makeup, and this manufacturing method is for manufacturing said die for hot working.

TECHNICAL FIELD

The present invention relates to a steel for a hot working die, a diefor hot working and a manufacturing method for the same.

BACKGROUND ART

In recent years, for the purpose of weight reduction and collisionsafety improvement of cars, a need for an ultrahigh strength steel sheethaving a tensile strength of higher than 1 GPa has been intensifying.However, when an attempt is made to form a steel sheet having a tensilestrength of 1.2 GPa or higher by hot pressing, an increase in theforming load or spring back and a problem with formability or the likeare caused. Therefore, recently, a hot stamping (also referred to as hotpressing or hot stamping) method has been gaining attention. In the hotstamping method, a steel sheet is heated to an austenite temperature orhigher and then pressed, a die is held at the bottom dead center, andthe steel sheet is rapidly cooled and quenched.

An advantage of the hot stamping method is that a formed product of anultrahigh strength steel sheet having a tensile strength ofapproximately 1.5 GPa can be obtained by quenching through die quenchingwhere the steel sheet is rapidly cooled in a die. In addition, anotheradvantage is that the formability is so excellent that spring backrarely occurs.

However, the hot stamping method has a problem of poor productivity.That is, time is required to hold the bottom dead center for diequenching or the like, and thus the productivity becomes poor. As ameasure for this, a die having high thermal conductivity is in demand.This is because, during die quenching, heat from a steel sheet isabsorbed into a die; however, as the thermal conductivity of the diebecomes higher, the time taken to hold the die at the bottom dead centerbecomes shorter, and the productivity becomes more favorable. Inaddition, dies for hot stamping are required to have high hardness inorder to enhance wear resistance, and steels for hot stamping dies arerequired to have both high hardness and high thermal conductivity whenmade into dies.

In the fields of hot forging or die casting as well, there is a tendencythat steels for dies having both high thermal conductivity and highhardness as described above are required to achieve the extension of theservice lives of dies or additional improvement in productionefficiency. Ordinarily, the amount of an alloy in a steel for a dieneeds to be increased to obtain a die having high hardness, but anincrease in the amount of an alloy leads to a problem of a decrease inthe thermal conductivity of the die, and hardness and thermalconductivity have a trade-off relationship. Therefore, studies areunderway to obtain an optimal compositional makeup by controlling theamount of an alloy. For example, in Patent Literature 1 and PatentLiterature 2, a compositional makeup of a steel for a die having bothhardness and thermal conductivity has been proposed. In addition, PatentLiterature 3 and Patent Literature 4 also disclose hot work tool steelsthat are useful as a material for dies that are used in warm and hotpressing, die casting, warm and hot forging and the like, have excellentthermal conductivity and also have excellent wear resistance.

CITATION LIST Patent Literature Patent Literature 1

-   Japanese Patent Laid-Open No. 2017-43814

Patent Literature 2

-   Japanese Patent Laid-Open No. 2018-24931

Patent Literature 3

-   Japanese Patent No. 5744300

Patent Literature 4

-   Japanese Patent Laid-Open No. 2017-53023

SUMMARY OF INVENTION Technical Problem

The steels for a die of Patent Literature 1 and 2 and the hot work toolsteels of Patent Literature 3 and 4 are useful inventions that enablehardness and thermal conductivity to be increased. However, when thequenching and tempering characteristics of steels for dies or hot worktool steels, the fact that the work surface of a die for hot stamping orthe like is used after a nitriding treatment and the like are taken intoaccount, there have been cases where conventional steels for a die orhot work tool steels lack hardness. Specifically, in recent years, therehas been a demand for a steel for a die capable of achieving a highhardness of 52 HRC or higher as a die for hot stamping or the like, buta high hardness of 52 HRC or higher cannot be stably obtained withPatent Literature 1-3. In addition, since the tempering temperaturewhere the highest hardness of a steel for a die can be obtained isordinarily near 575° C., if the highest hardness of a steel for a diedoes not reach 52 HRC, the hardness of a die further decreases fromlower than 52 HRC due to the nitriding treatment or an increase intemperature during use.

An objective of the present invention is to provide a steel for a hotworking die having both higher hardness and higher thermal conductivitythan ever and enabling the production of a die where the hardness ismaintained, a die for hot working and a manufacturing method for thesame.

Solution to Problem

In consideration of such circumstances, the present inventor found acompositional makeup that allows the control of the amount of an alloy,is capable of achieving high hardness and high thermal conductivity andis capable of maintaining the achieved high hardness (that is, thesoftening resistance is large) and reached a steel for hot working dieof the present invention. In addition, the present inventor found a diefor hot working capable of achieving high hardness and high thermalconductivity and also having excellent softening resistance by the useof the steel for a die and a manufacturing method for the same.

That is, an aspect of the present invention is a steel for a hot workingdie having a compositional makeup containing, in mass %, 0.45-0.65% ofC, 0.1-0.6% of Si, 0.1-2.5% of Mn, 1.0-6.0% of Cr, 1.2-3.5% of (Mo+½W)where Mo and W are contained independently or in combination, 0.1-0.5%of V, 0.15-0.6% of Ni, 0.1-0.6% of Cu, and 0.1-0.6% of Al or less, abalance being Fe and inevitable impurities.

Preferably, when the steel has been tempered at 575° C., a hardness is52 HRC or higher.

Another aspect of the present invention is a die for hot working havinga compositional makeup containing, in mass %, 0.45-0.65% of C, 0.1-0.6%of Si, 0.1-2.5% of Mn, 1.0-6.0% of Cr, 1.2-3.5% of (Mo+½W) where Mo andW are contained independently or in combination, 0.1-0.5% of V,0.15-0.6% of Ni, 0.1-0.6% of Cu, and 0.1-0.6% of Al or less, a balancebeing Fe and inevitable impurities.

Preferably, a hardness is 52 HRC or higher, and a thermal conductivityis 25 W/(m·K) or higher. Preferably, the die for hot working has anitrided layer on a work surface.

Still another aspect of the present invention is a manufacturing methodfor a die for hot working, in which the steel for a hot working die isquenched at a quenching temperature of 1020-1080° C. and tempered at atempering temperature of 540-620° C.

Preferably, after the quenching and the tempering, a nitriding treatmentis further carried out on a work surface.

Advantageous Effects of Invention

According to the present invention, a steel for a die optimal for hotworking can be obtained. In addition, the use of this steel for a diemakes it possible to provide a die for hot working having both highhardness and high thermal conductivity and maintaining the high hardnessand a manufacturing method for the same.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph figure showing hardness at each tempering temperatureof a steel for a die of each of the present invention examples andcomparative examples quenched and then tempered at 500-650° C.

FIG. 2 is a graph figure showing thermal conductivity of the steel for adie of each of the present invention examples and the comparativeexamples quenched and then tempered to hardness of 45-52 HRC.

DESCRIPTION OF EMBODIMENTS

A feature of the present invention is that it was found that there is acompositional makeup of a steel for a die optimal for achieving highhardness and high thermal conductivity of a die for hot working at thesame time when the fact that a die for hot working is produced byquenching and tempering a steel for a die or by carrying out a nitridingtreatment on a work surface thereof is taken into account. Inparticular, the feature is that it was found that there is an optimalmakeup for achieving a high hardness of 52 HRC or higher and highthermal conductivity of 25 W/(m·K) or higher at the same time.

In addition, the feature is that optimal quenching and temperingconditions for achieving high hardness and high thermal conductivity atthe same time in this optimal compositional makeup of a steel for a diewas found. In particular, the feature is that, when the temperingtemperature is set within a temperature range of 540-620° C. (preferablyto near 575° C.), a steel for a die of the present invention having theoptimal compositional makeup is capable of achieving a high hardness of52 HRC or higher, and thus a die is not easily softened (the degree ofsoftening is small) in spite of a subsequent nitriding treatment or atemperature-rising environment during use.

A die for hot working of the present invention can be applied to, forexample, a die for hot forging, a die for die casting, a hot extrusiondie and a die for hot stamping and is preferably applied to, inparticular, a die for hot stamping. Hereinafter, each configurationcondition of the present invention will be described.

The steel for a hot working die of the present invention has acompositional makeup containing, in mass % (hereinafter, simplyexpressed as “%”), 0.45-0.65% of C, 0.1-0.6% of Si, 0.1-2.5% of Mn,1.0-6.0% of Cr, 1.2-3.5% of (Mo+½W) where Mo and W are containedindependently or in combination, 0.1-0.5% of V, 0.15-0.6% of Ni,0.1-0.6% of Cu, and 0.1-0.6% of Al, a balance being Fe and inevitableimpurities.

C: 0.45-0.65%

C is an element that forms a solid solution in a basis material (matrix)by quenching to improve the hardness of dies. In addition, C is anelement that forms a carbide with a carbide-forming element such as Cr,Mo or V, which will be described below, to improve the hardness of dies.However, when the amount of C is too large, the toughness of diesdeteriorates due to the coarsening of a primary carbide or the like.Therefore, the amount of C is set to 0.45-0.65%. The amount of C ispreferably 0.47% or more. The amount of C is more preferably 0.49% ormore. In addition, the amount of C is preferably 0.63% or less. Theamount of C is more preferably 0.60% or less. The amount of C is stillmore preferably 0.58% or less.

Si: 0.1-0.6%

Si is used as a deoxidizing agent in a smelting step. In addition, Si isan element that forms a solid solution in the basis material to improvethe hardness of dies. However, when Si is too large, after smelting, asegregation tendency in steel becomes strong, and a solidified structurealso becomes coarse, which leads to the toughness deterioration of dies.In addition, Si is an element that significantly decreases the thermalconductivity of dies after quenching and tempering. Therefore, theamount of Si is set to 0.1-0.6%. The amount of Si is preferably 0.14% ormore. The amount of Si is more preferably 0.17% or more. In addition,the amount of Si is preferably 0.45% or less and more preferably 0.4% orless. The amount of Si is still more preferably 0.35% or less. Theamount of Si is far still more preferably 0.3% or less.

Mn: 0.1-2.5%

Mn is used as a deoxidizing agent or a desulfurization agent in thesmelting step. In addition, Mn is an element that contributes to thestrengthening of the basis material and improvement in hardenability andtoughness after quenching and tempering. However, when Mn is too large,the thermal conductivity of dies significantly decreases. Therefore, theamount of Mn is set to 0.1-2.5%. The amount of Mn is preferably 0.15% ormore. In addition, the amount of Mn is preferably 1.0% or less. Theamount of Mn is more preferably 0.35% or less. The amount of Mn is stillmore preferably 0.32% or less. The amount of Mn is far still morepreferably 0.3% or less.

Cr: 1.0-6.0%

Cr is an element that forms a solid solution in the basis material toincrease hardness. In addition, Cr is an element that increases hardnessby forming a carbide and an element that, similar to Mo and V, whichwill be described below, contributes to secondary curing duringtempering. Particularly, Cr is an element capable of increasingtempering softening resistance (capable of decreasing the decrease rateof hardness obtained by secondary curing even when the temperingtemperature is set to be high) compared with Mo and V. Normally, diesare adjusted to operation hardness by carrying out quenching andtempering on a steel for the dies, and thus an increase in the temperingtemperature is effective for increasing the thermal conductivity of diesfor hot working. In addition, in the present invention, when the Crcontent is set to 1.0% or more, hardness of 52 HRC or higher can beachieved even in a case where the tempering temperature is high, and adie for hot working having thermal conductivity of 25 W/(m·K) or highercan be obtained. In addition, it is also possible to obtain a die forhot working that maintains the above-described hardness and,furthermore, improves in thermal conductivity up to 28 W/(m·K) orhigher. The hardness and thermal conductivity are values measured atroom temperature (normal temperature).

In addition, when the Cr content is set to be large, since the nitridingcharacteristics of the steel for a die can be improved, it is possibleto improve the wear resistance of dies (hardness of work surfaces) whilemaintaining the hardness of the dies due to the improvement in thesoftening resistance by, for example, further carrying out a nitridingtreatment on the work surfaces of the quenched and tempered dies.

However, when the Cr content is too large, it becomes difficult toincrease the thermal conductivity of dies due to the fact itself thatthe amount of an alloy in the steel for a die becomes large. Therefore,the amount of Cr is set to 1.0-6.0%. The amount of Cr is preferably 1.5%or more. The amount of Cr is more preferably 2.0% or more. In addition,the amount of Cr is preferably 5.5% or less, more preferably 4.8% orless and still more preferably less than 4.5%. In addition, in a casewhere emphasis needs to be placed on improvement in, particularly, thethermal conductivity, the amount of Cr can also be set to 4.0% or lessor 3.5 or less.

(Mo+½W) where Mo and W are Contained Independently or in Combination:1.2-3.5%

Mo and W are, similar to Cr, elements that form a solid solution in thebasis material to increase hardness, in addition, elements that increasehardness by forming a carbide and elements that contribute to secondarycuring during tempering. In addition, Mo and W are elements that improvehardenability. Since W is approximately twice the atomic weight of Mo,can be regulated as (Mo+½W) (it is needless to say that only any one maybe added or both can be added). However, when the Mo or W content is toolarge, the thermal conductivity of dies become low due to the factitself that the amount of an alloy in the steel for a die becomes large.Therefore, Mo and W are set to 1.2-3.5% in terms of the Mo-equivalentrelational formula of (Mo+½W). The amount of Mo and W is preferably 1.5%or more. The amount of Mo and W is more preferably 1.7% or more. Theamount of Mo and W is still more preferably 1.9% or more. In addition,the amount of Mo and W is preferably 3.4% or less. The amount of Mo andW is more preferably 3.2% or less.

In the case of the present invention, since W is an expensive element,it is possible to replace all of W with Mo. At this time, Mo: 1.2-3.5%is satisfied (which is also true for the preferable ranges). However, Wcan be contained as an impurity.

V: 0.1-0.5%

V is, similar to Cr, an element that increases hardness by forming acarbide and an element that contributes to secondary curing duringtempering. However, when the amount of V is too large, the thermalconductivity of dies become low due to the fact itself that the amountof an alloy in the steel for a die becomes large. Particularly, in thepresent embodiment, since the thermal conductivity tends to become lowdue to an influence of Ni, Cu and Al added to improve the strengthcharacteristics of dies as described below, it is important to limit Vto 0.1-0.5% to satisfy both high thermal conductivity and a highhardness characteristic. The amount of V is preferably 0.2% or more. Inaddition, the amount of V is preferably 0.45% or less and morepreferably 0.4% or less.

Ni: 0.15-0.6%

Ni is an element that contributes to toughness improvement of dies. Inaddition, in the present embodiment, Ni bonds to Al to form andprecipitate a Ni—Al-based intermetallic compound and is capable ofimproving the strength characteristic of the steel for a die bysecondary curing. However, when the amount of Ni is too large, sincethere is a possibility that the thermal conductivity may becomesignificantly low due to an increase in the amount of an alloy in thesteel for a die, the amount of Ni is set to 0.15-0.6%. The amount of Niis preferably 0.2% or more. In addition, the amount of Ni is preferably0.5% or less and more preferably 0.45% or less.

Cu: 0.1-0.6%

Cu is also an element that, similar to Ni, bonds to Al to form andprecipitate an intermetallic compound and is capable of improving thestrength characteristic of the steel for a die by secondary curing.However, when the amount of Cu is too large, similar to Ni, the thermalconductivity of dies become low due to the fact itself that the amountof an alloy in the steel for a die becomes large. Therefore, the amountof Cu is set to 0.1-0.6%. The amount of Cu is preferably 0.2% or more.In addition, the amount of Cu is preferably 0.5% or less and morepreferably 0.45% or less.

Al: 0.1-0.6% or Less

As described above, Al bonds to Ni or Cu to form an intermetalliccompound. When this Al content is too small, since the intermetalliccompound is not sufficiently formed, the strength improvement effectcannot be obtained, and, on the other hand, when the Al content is toolarge, there is a possibility that the thermal conductivity of dies maysignificantly decrease. Therefore, the amount of Al is set to 0.1-0.6%.The amount of Al is preferably 0.2% or more. In addition, the amount ofAl is preferably 0.5% or less and more preferably 0.4% or less.

Furthermore, in the present embodiment, in order to form and precipitatethe intermetallic compound in appropriate quantities, Ni/Al ispreferably 1.0-2.0. A more preferably upper limit of Ni/Al is 1.7, and astill more preferable upper limit is 1.5. Alternatively, furthermore, inthe present embodiment, in order to form and precipitate theintermetallic compound in appropriate quantities, Cu/Al is preferably1.0-2.0. A more preferably upper limit of Cu/Al is 1.7, and a still morepreferable upper limit is 1.5.

Balance being Fe and Inevitable Impurities

When the fact that an increase in the amount of an alloy in the steelfor a die decreases the thermal conductivity of dies is taken intoaccount, it is preferable that the balance other than theabove-described elemental species is substantially Fe. Elemental speciesthat are not clearly mentioned here (for example, elemental species suchas P, S, Ca, Mg, O (oxygen) and N (nitrogen)) are elements that possiblyremain in steel inevitably, and these elements are allowed to becontained as impurities. At this time, when P is too large, P issegregated in prior austenite grain boundaries during a heat treatmentsuch as tempering, and the toughness of dies deteriorates. Therefore,the amount of P is preferably regulated to be 0.05% or less. The amountof P is more preferably regulated to be 0.03% or less. In addition, whenS is too large, hot workability deteriorates at the time of blooming aningot or the like. Therefore, S is preferably regulated to be 0.01% orless. The amount of S is more preferably regulated to be 0.008% or less.

When a steel for a die having the above-described compositional makeupis quenched and tempered, it is possible to obtain the die for hotworking of the present invention that is excellent in terms of hardnessand thermal conductivity. The hardness of the die for hot working of thepresent invention is a value measured at room temperature (normaltemperature), and it is possible to achieve sufficient hardness, forexample, 52 HRC or higher and to impart excellent wear resistance to thedie. In addition, the hardness of the die can be preferably set to 53HRC or higher by adjusting the tempering temperature.

In the present invention, there is no need to regulate the upper limitof the hardness of the die. However, in the case of steels for a diehaving the above-described compositional makeup, the upper limit is,realistically, approximately 60 HRC based on the peak hardness ofsecondary curing (roughly within a tempering temperature range of540-620° C.). In addition, regardless of the fact that the peak hardnessof secondary curing is approximately 60 HRC, the upper limit of thishardness is preferably set to 58 HRC or lower since, that is, thetempering temperature can be raised beyond the peak hardness (that is,the thermal conductivity can be increased).

In addition, when a steel for a die having the above-describedcompositional makeup is quenched and tempered to adjust the hardness ofa die to 52 HRC, the thermal conductivity of the die of the presentembodiment is 25 W/(m·K) or higher. This thermal conductivity is a valuemeasured at room temperature (normal temperature). The thermalconductivity is preferably 28 W/(m·K) or higher. In a case where it isnecessary to further increase the thermal conductivity of the die of thepresent invention having the above-described thermal conductivity, thethermal conductivity can be further increased by setting the hardness toless than 52 HRC. In addition, it is also possible to adjust thehardness of the die to higher than 52 HRC once the die has sufficientthermal conductivity. Specifically, when the hardness of the die is 45HRC or higher and 48 HRC or lower, the thermal conductivity ispreferably 30 W/(m·K) or higher, more preferably 32 W/(m·K) or higherand still more preferably 34 W/(m·K) or higher. In addition, when thehardness of the die is 53 HRC or higher and 55 HRC or lower, the thermalconductivity is preferably 25 W/(m·K) or higher and more preferably 27W/(m·K) or higher.

Such a die can be achieved with a steel for a die having a heattreatment characteristic that exhibits hardness of 52 HRC or higher whentempered at 575° C. At the time of confirming this heat treatmentcharacteristic, the quenching temperature before tempering can be setto, for example, 1030° C. In addition, the steel for a die of thepresent invention has the above-described heat treatment characteristic.Therefore, in dies that are being used in, for example, the hot stampingmethod (for example, 100-400° C.), it is possible to maintain highhardness, and it is also possible to maintain high thermal conductivity.

In the case of the present invention, there is no need to specify theupper limit of the thermal conductivity of the die. However, when thefact that the hardness of the die is decreased by increasing thetempering temperature (for example, adjusting the tempering temperatureto a temperature of higher than 600° C.) is taken into account, theupper limit is, realistically, approximately 50 W/(m·K). The upper limitis preferably 47 W/(m·K) or lower. The upper limit is more preferably 45W/(m·K) or lower. In addition, when the hardness of the die ismaintained at 52 HRC or higher, the upper limit of the thermalconductivity is, realistically, approximately 40 W/(m·K). The upperlimit is preferably 38 W/(m·K) or higher.

The die for hot working of the present invention preferably has anitrided layer on a work surface.

As described above, the die for hot working of the present invention hasboth high hardness and high thermal conductivity. In addition, when thisdie further has a nitrided layer on the work surface, it is possible tofurther improve the wear resistance of the die (the hardness of the worksurface). In addition, it is also possible to suppress a decrease in thehardness of the die main body at the time of a nitriding treatment dueto the quenching and tempering characteristics of the steel for a die ofthe present invention. The work surface refers to a surface of the diethat comes into contact with a workpiece during hot working.

In a manufacturing method for a die for hot working of the presentinvention, the steel for a die is quenched and tempered.

When the steel for a die having the above-described compositional makeupis quenched and tempered, the quenching temperature varies with thetarget hardness or the like and can be set to, for example,approximately 1020-1080° C. The quenching temperature is preferably1050° C. or lower.

In addition, the steel for a die quenched at this quenching temperatureis tempered at a tempering temperature of, for example, 540-620° C.,whereby it is possible to obtain a die having thermal conductivity of 25W/(m·K) or higher while stably achieving hardness of 52 HRC or higher.At this time, the upper limit of the tempering temperature is preferablyset to approximately 600° C. on the condition that hardness of 52 HRC orhigher is maintained. The upper limit is more preferably 595° C. orlower. The upper limit is still more preferably 590° C. or lower. Inaddition, the lower limit of the tempering temperature is preferably setto approximately 550° C. The lower limit is more preferably 555° C. orhigher. The lower limit is still more preferably 560° C. or higher.

When a heat treatment characteristic enabling hardness of 52 HRC orhigher to be achieved at a tempering temperature near 575° C., at whichthe peak hardness of secondary curing is exhibited, is regarded as astandard, the steel for a die of the present invention satisfying thisstandard of the heat treatment characteristic is capable of maintaininghardness of 45 HRC or higher even in a broad tempering temperature rangeof 540-620° C. In addition, when the above-described peak hardness ishigher than 52 HRC, for example, 53 HRC or higher, 54 HRC or higher or55 HRC or higher, it is possible to maintain hardness of 52 HRC orhigher in a broad tempering temperature range. In addition, it ispossible to obtain thermal conductivity of 25 W/(m·K) or higher in abroad tempering temperature, which makes it possible to improve thethermal conductivity at tempering temperatures of, particularly, 575° C.or higher.

The steel for a die of the present invention can be made into a die forhot working having predetermined hardness by quenching and tempering. Inaddition, during these processes, the steel for a die can be made into ashape of the die for hot working by a variety of machining such ascutting or perforation. Regarding the timing of this machining, themachining can be carried out in a state where the hardness is low beforequenching and tempering (that is, in an annealed state). In addition, inthis case, finishing may also be carried out after quenching andtempering. In addition, depending on the situation, the above-describedmachining may be carried out together with the finishing in apre-hardened state after quenching and tempering.

In the manufacturing method for a die for hot working of the presentinvention, preferably, a nitriding treatment is further carried out onthe work surface of the die on which the above-described quenching andtempering have been carried out.

As described above, when the steel for a die having the above-describedcompositional makeup is quenched and tempered, it is possible to obtaina die having thermal conductivity of 25 W/(m·K) or higher when, forexample, the hardness has been adjusted to 52 HRC. In addition, sincethe steel for a die having the above-described compositional makeup isalso excellent in terms of a nitriding characteristic, when a nitridingtreatment is further carried out on the work surface of the die that hasbeen quenched and tempered as described above, it is possible to improvethe wear resistance of the die (the hardness of the work surface). Inaddition, it is also possible to suppress a decrease in the hardness ofthe die main body at the time of the nitriding treatment due to thequenching and tempering characteristics of the steel for a die of thepresent invention. At this time, as the conditions for the nitridingtreatment, for example, conditions for a variety of known nitridingtreatments such as a gas nitriding treatment or a salt bath nitridingtreatment can be applied.

Example 1

Ten-kilogram ingots having a compositional makeup in Table 1 weresmelted. In addition, these ingots were heated to 1160° C., cogged witha hammer and then left to be cooled, an annealing treatment was carriedout on these cooled steel materials at 870° C., thereby producing steelsNos. 1-6 that were present invention examples and steels Nos. 7-9 thatwere comparative examples.

TABLE 1 Specimen No. C Si Mn Cr Mo V Ni Cu Al Fe* Note No. 1 0.55 0.200.35 2.10 3.20 0.40 0.45 0.30 0.34 Bal. Present No. 2 0.55 0.20 0.352.10 3.20 0.40 0.30 0.30 0.25 Bal. Invention No. 3 0.55 0.20 0.35 2.103.20 0.40 0.15 0.13 0.11 Bal. Example No. 4 0.60 0.25 0.25 1.70 2.000.40 0.30 0.30 0.25 Bal. No. 5 0.55 0.25 0.25 1.70 3.20 0.40 0.30 0.300.25 Bal. No. 6 0.60 0.25 0.25 1.70 3.20 0.40 0.30 0.30 0.25 Bal. No. 70.55 0.20 0.35 2.10 3.20 0.40 <0.10 <0.10 <0.10 Bal. Comparative No. 80.55 0.25 0.25 1.70 2.00 0.40 <0.10 <0.10 <0.10 Bal. Example No. 9 0.550.25 0.25 1.70 3.20 0.40 <0.10 <0.10 <0.10 Bal. *P ≤ 0.05%, S ≤ 0.01%

<Evaluation of Tempered Hardness>

The steels for a die Nos. 1-9 were quenched at a quenching temperatureof 1030° C. At this time, as cooling conditions, cooling rates when thesteels for a die, which are referred to as present invention steels andcomparative steels, were as large as an actual die for hot stamping wereassumed, and the half cooling time was set to 40 minutes (the halfcooling time refers to a time required to cool a workpiece from thequenching temperature to a temperature of (quenching temperature+roomtemperature)/2). In addition, the quenched steels for a die weretempered at tempering temperatures of 500-650° C. The tempering wascarried out twice, and the steels were held at each temperature for twohours. The tempering temperatures were set to a total of sevenconditions at 25° C. intervals. In addition, for each of Nos. 1-9,Rockwell hardness (C scale) of the central part at room temperature wasmeasured at each tempering temperature. The results are shown in FIG. 1to FIG. 3 .

Nos. 1-7, which were the present invention examples, achieved thetempered hardness of 52 HRC or higher within a tempering temperaturerange of 550-600° C. In contrast, Nos. 7-9, which were the comparativeexamples, all had tempered hardness of lower than 52 HRC within atempering temperature range of 500-650° C.

<Evaluation of Thermal Conductivity>

Subsequently, the thermal conductivity of Nos. 1-9 was measured. Thetempered hardness values of specimens at the time of measuring thethermal conductivity were 52 HRC for Nos. 1 to 6, which were the presentinvention examples, and 51 HRC, 50 HRC and 45 HRC for Nos. 7, 8 and 9,which were the comparative examples, respectively. Regarding themeasurement procedure, first, dies were worked into test pieces having adisc shape that was 10 mm in diameter and 2 mm in thickness, and thethermal diffusivity and specific heat of these test pieces were measuredby the laser flash method. In addition, the thermal conductivity at roomtemperature was calculated from the following formula using the valuesof the measured thermal diffusivity and specific heat. The results areshown in FIG. 2 .

Thermal conductivity λ (W/(m·K))=ρ·α·C _(p)

(ρ: density at room temperature, α: heat diffusivity, C_(p): specificheat)

From the results of FIG. 2 , it was confirmed that Nos. 1 to 6, whichwere the present invention examples, achieved thermal conductivity of 25W/(m·K) or higher. On the other hand, Nos. 7 to 9, which were thecomparative examples, had the same level of thermal conductivity as thepresent invention, but originally had peak hardness that did not reach52 HRC. Therefore, when thermal conductivity is adjusted (specifically,when the tempering temperature is increased to increase thermalconductivity) in steels for a die like the comparative examples,hardness further decreases, and it is not possible to cope with dieswith a variety of required characteristics. In contrast, it was possibleto confirm that the steels for a die of the present invention exampleshad not only sufficient peak hardness but also excellent temperingsoftening resistance and thus the present invention examples having bothhigh thermal conductivity and a high hardness characteristic hadadvantageous characteristics in uses for dies for hot working.

1. A steel for a hot working die having a compositional makeup containing, in mass %, 0.45-0.65% of C, 0.1-0.6% of Si, 0.1-2.5% of Mn, 1.0-6.0% of Cr, 1.2-3.5% of (Mo+½W) where Mo and W are contained independently or in combination, 0.1-0.5% of V, 0.15-0.6% of Ni, 0.1-0.6% of Cu, and 0.1-0.6% of Al, a balance being Fe and inevitable impurities.
 2. The steel for a hot working die according to claim 1, wherein, when the steel has been tempered at 575° C., a hardness is 52 HRC or higher.
 3. A die for hot working having a compositional makeup containing, in mass %, 0.45-0.65% of C, 0.1-0.6% of Si, 0.1-2.5% of Mn, 1.0-6.0% of Cr, 1.2-3.5% of (Mo+½W) where Mo and W are contained independently or in combination, 0.1-0.5% of V, 0.15-0.6% of Ni, 0.1-0.6% of Cu, and 0.1-0.6% of Al, a balance being Fe and inevitable impurities.
 4. The die for hot working according to claim 3, wherein a hardness is 52 HRC or higher, and a thermal conductivity is 25 W/(m·K) or higher.
 5. The die for hot working according to claim 3, comprising: a nitrided layer on a work surface.
 6. A manufacturing method for a die for hot working, wherein the steel for a hot working die according to claim 1 is quenched at a quenching temperature of 1020-1080° C. and tempered at a tempering temperature of 540-620° C.
 7. The manufacturing method for a die for hot working according to claim 6, wherein, after the quenching and the tempering, a nitriding treatment is further carried out on a work surface.
 8. The die for hot working according to claim 4, comprising: a nitrided layer on a work surface.
 9. A manufacturing method for a die for hot working, wherein the steel for a hot working die according to claim 2 is quenched at a quenching temperature of 1020-1080° C. and tempered at a tempering temperature of 540-620° C.
 10. The manufacturing method for a die for hot working according to claim 9, wherein, after the quenching and the tempering, a nitriding treatment is further carried out on a work surface. 